A conventional pulse radar apparatus comprises a high-frequency transmitting section configured to generate a pulsed transmission signal by modulating a high-frequency carrier wave and by cutting out a carrier frequency only for a very short time, a transmitting antenna configured to radiate the transmission signal generated by the high-frequency transmitting section to the air as radio wave, a receiving antenna configured to receive reflected waves of the radio wave radiated from the transmitting antenna and returned by being reflected by an object, a high-frequency receiving section configured to input the received signal from the receiving antenna to down-convert to a baseband signal, and a baseband section configured to input the baseband signal from the high-frequency receiving section to calculate a distance to the object and others.
The high-frequency transmitting section further includes an oscillator configured to generate the carrier wave having a predetermined frequency, a switch configured to cut out the carrier wave generated by the oscillator as pulsed carrier waves, and others. The high-frequency receiving section also includes a correlator configured to correlate the transmission signal with the received signal and an IQ mixer configured to down-convert an output signal of the correlator into the baseband signal. The baseband section includes an amplifier configured to amplify the baseband signal received from the high-frequency receiving section, an A/D converting unit configured to convert the signal amplified by the amplifier into a digital signal, a digital signal processing unit configured to process the digital signal received from the A/D converting unit to calculate the distance to and relative speed of the object, and a control unit configured to control the pulse radar apparatus. The control unit is configured to control ON/OFF of the switch of the high-frequency transmitting section and of the correlator of the high-frequency receiving section.
As described above, the pulse radar apparatus includes the high-frequency transmitting and receiving sections (both referred to as the “RF section” hereinafter) that are configured to process high-frequency signals, and the baseband section configured to process low-frequency signals. Because it is necessary to use an expensive substrate capable of accommodating with high frequency for the RF section among them, it has been common to dispose only the RF section on the substrate capable of accommodating with high frequency and to dispose the baseband section on an inexpensive substrate to lower costs. Still further, a small and inexpensive multi-pin connector is used since the past as a means for connecting the RF section with the baseband section disposed respectively on the separate substrates.
When the baseband section and the RF section which are formed on the separate substrates are connected by the inexpensive intensive multi-pin connector, there arises a problem that interference noise signals leak into the received signal. Thus, there is a problem that the desirable received signal is buried in the interference noise signals when sufficient receiving strength cannot be obtained if the unwanted wave such as a control signal secondarily generated leaks into the received signal and turns out to be the interference noise signal in the multi-pin connector. Then, the prior art multi-pin connector is arranged so as to able to detect even a received signal whose receiving strength is small by increasing isolation of the multi-pins as much as possible to reduce an amount of the interference noise signals.
Such interference noise signals exist also in various radar apparatuses, though its cause is different, and there are technologies for removing such interference noise signals. Patent Document 1 discloses a process for reducing interference noise signals of stationary noise components superimposed on a received signal (noise component whose temporal fluctuation of frequency and level is small) in a FM-CW radar. The radar stores the stationary noise components and detects an object after subtracting the noise components from a distribution of spectrum of the received signal.
Meanwhile, a pulse radar apparatus configured to process a signal by dividing a received signal into I phase and Q phase has such a problem that a difference of gains is produced due to individual differences of an I-phase-side unit and a Q-phase-side unit because gains of amplifiers operated in parallel in the baseband section and values of sampling implemented in the A/D converting unit are not equal. If such individual difference is produced, errors contained in respective measured values increase in measurements of azimuth (angle measurement), of distance (distance measurement) and of relative speed of an object, which are purports of the radar apparatus.
Patent Document 2 discloses a means configured to correct such errors of the gains. In order to correct the difference of gains produced between two or more receiving digital signals, Patent Document 2 proposes a system comprising a storage unit configured to store an initial difference of gains caused by characteristic variation of units composing each receiving system, a detecting unit configured to detect the difference of gains per receiving system, a correction-amount calculating unit configured to generate a gain-difference correcting signal based on the initial difference of gains and the difference of gains in operation, and an arithmetic processing unit configured to correct the gain of output of A/D conversion based on the gain-difference correcting signal. The system calculates a correction value by actually radiating radar waves.
Still further, in order to be able to detect weak reflected waves from a distant object, the pulse radar apparatus amplifies the baseband signal by a fixed gain amplifier having a high gain. Due to that, there are such problems that reflected wave from an object located in a short distance is amplified to a high level and exceeds a maximum input voltage of the A/D converting unit. If the output signal from the A/D converting unit exceeds the maximum input voltage as described above, the digital signal processing unit is unable to correctly calculate a distance to the object, relative speed of the object and azimuth angle. Then, Patent Document 3 uses a variable gain amplifier, instead of the fixed gain amplifier, to be able to adjust the gain corresponding to strength of the received signal.
When the variable gain amplifier is used, there arises such a problem that noise is generated in an output when the gain is changed over discontinuously. Patent Document 3 describes such a problem that a slice level detecting circuit in a later stage outputs what is different from an original slice level due to such noise, thus aggravating a bit error rate. In order to solve such problem, Patent Documents 3 is provided with a slice level fixing circuit configured to fix the slice level when the gain of the variable gain amplifier is changed over. Patent Document 3 prevents the aggravation of the bit error rate by reducing an influence of the noise when the gain is changed over by thus arranging so that the circuits of the latter stage are not influenced.